I owe this entry to Scientific American, where they have a great article up on this very subject. Imitation being the most sincere form of flattery, I'm getting me some of this action! Jesse Bering has an excellent write up over there, far more in depth than mine, but I flatter myself that mine has better pictures.

And it's a great subject. I mean, we all probably know, deep down (or not so deep down), that penises are some funny looking things. All bulbous at the top and weird. You know it's true. But what if the penis was shaped all funny...for a reason? Gallup, GG. "The human penis as a semen displacement device." Evolution and Human Behavior, 2003.
(WARNING: pictures of penises below the fold. Mostly not human, but you're still warned.)

And the best thing about this paper, it's another example of those not afraid to develop a coital simulator in the name of SCIENCE!
And this is a great theme for the First of May (NSFW, like everything else in this post...):

Sci will admit she's had a great time writing this post. I mean, she usually has a great time with Friday Weird Science, but this one especially so. I mean, take a look at THIS:
(Courtesy of awesome blogger Ed Yong)
And this!!!
(Via ABC Science News)
And especially this:
(From Neatorama)
I mean, what lady beetle WOULDN'T want that in her most sensitive parts? (Actually, I don't know whether lady beetles have sensitive vaginas, but for their sake, I hope not.)
Anyway, these beetles have some pretty amazing penises (penae? penii?). There's been a hypothesis out there (though it may have been disproven) that beetles have penises like maces to scoop the semen of other males OUT of the female (in case there is any), or, alternatively, to destroy the vaginal tract behind them, making sure that no other male beetle will be tapping that. It could be sexual preference on the part of the females. Beetles tend to woo with things like huge tasty balls of dung or nests or fancy calls and carapaces, but you never know, some could be kinky.
So we know that beetles (and some other insects, like fruit flies) are capable of displacing sperm from the other males that came before them. But what about the males of other species? What about...Humans?
In general, it's in a male human's best interest to ensure that the children he's raising are his, and he's not raising someone else's child by mistake. Evolutionary psychologists have listed lots of ways in which males ensure that the child in theirs. Other than the obvious paternity test, men guard their mates, and male and female humans look for a resemblance in a child to the father as a way to assess paternity. Not only that, if a man goes without seeing his chosen lady for a longer period of time (three days or more), his sperm counts are HIGHER than if he was seeing her every day, as the guy tries to hedge his bets. Finally, the last few drops of a guy's ejaculate actually have spermacide in them, to make sure that a guy coming next (heh heh) will have less of a chance. But what do you do if a rival has already gotten to your girl? You gotta get him out of there. Literally.
Relative to body size, human males have pretty large penises (don't get all egotistical, guys). Humans blow chimps out of the water with regard to penis size. Not only that, the human penis has a bulbous glans (the tip), and there is a ridge running perpendicular, along the penis shaft.
(Via wikipedia, and the ridge running perpendicular would be running along the bottom)
Gallup et al had a hypothesis that the enlarged glans of the penis is enlarged, and has a ridge extending perpendicular, in order to scoop previously placed semen out of the vagina and replace it with the semen of the current penis' owner.
But they had to test it. And getting an IRB approval for testing sex in the same woman with multiple partners sucks. Never mind trying to get the volunteers. They need to work with what they had.
And what they had: dildos!
Behold figure 1. On the far left, you can see the fake vagina they used. These are available in sex shops, and in this case they just tied off the end to "prevent spillage". The two tested dildos were B and D. B was a smaller size and had a less prominent ridge, while D had the most prominent ridge. The boring old dildo C was used as a control, with no glans or ridge.
For each trial, they inserted fake semen in an average amount (about 3mL) into the fake vagina. Then, after firmly bracing the fake vagina against some fiberglass, they inserted the penis of choice, thrust, and withdrew. As they withdrew, they pinched off the end of the fake vagina to make sure that semen didn't flow back, and looked at the percentage of semen that got displaced. They did this for all three models, and also check for thrusting depth, to see if depth (or length) had any effect).
Figure 2: the experiment in action. The piece of plastic being held perpendicular is to catch the semen as it gets scooped out.
And it turns out that it WORKED! The two morphologically correct dildos, B and D, each scooped out 90% of the semen!! Poor little plastic dildo C only got 35%, which is still a pretty good start. But it didn't happen quite the way they thought it would. The scientists hypothesized that the semen would be forced over the glans and along the coronal ridge. Instead, the semen was forced out on the underside of the penis, which seems like an easier route of passage anyway. It did pool at the base of the coronal ridge, and so the coronal ridge itself appears to be important. But the glans appears to be the most important. When they experimentally cut off the glans from a fake penis, semen removal dropped to below 40%. (NOTE: the glans does not refer to the foreskin, it's the bulbous area underneath. So no conclusions about circumcised vs uncircumcised can be drawn. No dildos with foreskins were tested.)
Depth of thrusting mattered, too. In fact it mattered a lot. Morphologically correct penises, when thrust fully into the vagina (almost to the depth of where the cervix would be) could displace 90% of the semen, but when thrust 3/4 of the way in, only displaced about 35%. Shallow thrusting was a total bust.
Of course, keep in mind that each thrust test was only one thrust. Repeated thrusting, even at variable depth, may be enough to displace most or all of the semen already there.
For the next experiment, the researchers wanted to see if scooping was the same with all kinds of "semen". So they made a normal batch of semen (18.8 g of sifted, white, unbleached flour mixed with 250 mL of water. This mixture was brought to a boil, simmered for 15 min while being stirred, and allowed to cool. This is science on the cheap.), a thick batch (0.147 g flour/mL water), and a thin batch (0.126 g flour/mL). You ever want to make fake semen, say you found it here. Though apparently the "sexually experienced" men of the group (remember, it WAS a group of scientists, so I suppose probable number of sexually experienced males to be n=1) described the thin semen as the best approximation. And it turns out that there was a significant effects of semen viscosity. Thicker semen wasn't scooped out as well as thin semen, perhaps because it was stickier.
So now we know that semen viscosity, depth of penetration, the bulbous glans, and the coronal ridge all affect how semen can be scooped. But this is all in a model. They needed some human data.
It turns out the only part of this you could really test without some major IRB issues would be the depth of penetration (though I imagine they could have brought some guys into the lab and measured viscosity, it wouldn't have helped without testing scoop). They issued a survey to college couples, asking two sets of questions:
1) In a relationship, after a woman has admitted to sexual infidelity, did you notice any change in intercourse?
2) When you have been separated from your partner for more than a few days, do you notice any difference in the next instance of coitus?
What they were looking for was changes in the depth of penetration, which would aid in scooping out semen. If men penetrated deeper after allegations of infidelity or separation, it would imply that they were trying (subconsciously) to displace the semen of a possible other man.
The first question returned some confused answers. Only about half the participants noticed any change. Of those that did, both males and females reported that men did indeed thrust deeper and faster than normal.
With regards to separation, 70% noticed a change in sexual behavior, and both sexes reported deeper and quicker thrusting.
Of course, there are some issues. The coital models they used aren't real men and women, though they did use several different kinds of fake penis, and a couple of different kinds of fake vagina, to get a good measurement.
There are also issues with the surveys. The length of time apart didn't take into account the amount of time since the male's last ejaculation, which can change the behavior of subsequent sexual encounters. Not only that, guys might think that deeper thrusting is just the thing to do after separation or infidelity, so there might be a cognitive bias, as opposed to the subconscious changes that the authors hypothesized taking place. But it's to the credit of the survey that both women and men reported similar changes in sex.
So what does all this means? It implies that the shape of the penis, with an enlarged glans and coronal ridge, could be beneficial for men in scooping out the semen of rival males. Not only that, it means that deeper thrusting may be more effective, and that deep thrusting, combined with a ridge and some viscous semen, could make a guy an unstoppable fertilizing force. And it implies that perhaps we need new slang terms for the penis. Instead of calling it your "rod" or "woody", perhaps you should call it your "scoop".Gallup, G. (2003). The human penis as a semen displacement device Evolution and Human Behavior, 24 (4), 277-289 DOI: 10.1016/S1090-5138(03)00016-3

WRT the barbed penises (penii?), I recall from my studies of cat reproduction that the barbs in that species stimulate ovulation. Obviously, the male who has intercourse closest to ovulation has an advantage, so better barbs are a selected trait.
WRT the circumcised/vs non-circumcised, uncut men are largely (so to speak) sliding around inside their own skin. I would bet (quite a bit, actually), that circumcision noticeably increases the scoop rate.

Ha! So, I suppose we are to believe that increased thrusting associated with climax is not related to (physical) excitement? And increased thrusting following separation couldn't be modeled from this normal phenomenon?
And if the response to infidelity can cover a continuum from immediate cessation of the relationship to renewed "vows," there's no bias built into the question by the fact that the couples who continued to mate were the ones that "made up"?
And of course, in all of this, the female is always passive. It couldn't be that females choose to mate with larger males because they like it?

Seconded.
This whole test is carried out on the back of darwins male=infidelity / female=monogamy theory which is being destroyed with modern tests and surveys. Not to mention the historical nature of humans is that we inhabited close knit colonies or tribes, and had a social system for the greater part of our existence, not really a place you'd imagine your bro sleeping with your girl. And even though that does happen, it's not a trope so prevalent to explain this evolutionary feature on a penis.
Besides, this test is also hoping that we all do missionary. And always have done. See, bonobos.

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All these fake penis for test purposes had no foreskin. It would be interesting to see what effect an intact foreskin has on this process. My guess would be in would make it much more efficient, as when the penis is withdrawn, the foreskin bunches up against the coronal ridge and forms a better seal to scope the semen out.